1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser diode (referred to also as a "DFB-LD", hereinafter) which realizes laser oscillation with a single wavelength. In particular, the present invention relates to a gain-coupled distributed feedback semiconductor laser diode (Gain-Coupled DFB-LD) which has a mechanism for providing a distributed feedback of light by a periodical distribution of gain.
2. Description of the Related Art
A distributed feedback semiconductor laser diode (DFB-LD) having an active layer for generating stimulated emission light includes a device structure where the refractive index, the gain, etc., for the stimulated emission light are periodically changed in the guiding direction of the stimulated emission light. In such a DFB-LD, the stimulated emission light is subject to an optical distributed feedback by the periodical change of the refractive index, the gain, etc., and laser oscillation is thereby obtained with a single wavelength.
A DFB-LD in which the distributed feedback is provided by the periodical change of refractive index (i.e., refractive index coupling) is called an index-coupled DFB-LD (referred to also as an "IC-DFB-LD", hereinafter). On the other hand, a DFB-LD in which the distributed feedback is provided by the periodical change of gain (i.e., gain coupling) is called a gain-coupled DFB-LD (referred to also as a "GC-DFB-LD", hereinafter). The IC-DFB-LD and the GC-DFB-LD are distinguished from each other.
Laser oscillation with a single wavelength can be obtained much more easily in the GC-DFB-LD than in the IC-DFB-LD, as shown in, for example, Journal of Applied Physics, vol. 43, page 2327 (1972). Moreover, the GC-DFB-LD has excellent characteristics which are practically important and which are not found in the IC-DFB-LD, such that a noise is not generated even when intense returning light is present. Furthermore, the setting of the reflectivity of the emission end face of a semiconductor laser diode is important for increasing the output and the efficiency of the semiconductor laser diode. In this regard, the reflectivity of the emission end face must be made substantially zero in the IC-DFB-LD, whereas it can be set to an arbitrary value in the GC-DFB-LD. Thus, the GC-DFB-LD has a great freedom in setting the end face reflectivity, and therefore is advantageous in optimizing device structures of semiconductor laser diodes.
With these various excellent characteristics, the GC-DFB-LD is very useful in practice as a single-wavelength light source for optical instruments, high speed optical transmission apparatuses, optical recording apparatuses, etc.
In order to realize such a GC-DFB-LD, it is necessary to incorporate in a semiconductor laser diode a structure for periodically changing the gain. For this purpose, two methods are generally employed.
One method is to form a structure in which the shape and properties of an active layer as well as a density of a current to be injected into the active layer are periodically changed (i.e., a gain-based diffraction grating) in a semiconductor laser diode. In this way, the active layer itself is provided with a periodical change of gain. The other method is to form a structure in which light-absorbing regions are periodically provided in the vicinity of an active layer which generates a uniform gain (i.e., a absorption-based diffraction grating). Due to such a structure, a periodical change of gain is effectively provided.
The basic structure for implementing the second method is disclosed in, for example, Japanese Patent Publication No. 6-7624. Hereinafter, the structure disclosed in this publication will be described with reference to FIG. 9.
FIG. 9 is a perspective view showing a structure of a GC-DFB-LD 900 disclosed in the above publication.
The GC-DFB-LD 900 has an un-AlGaAs (undoped-AlGaAs) active layer 905 for generating stimulated emission light. In the vicinity of the active layer 905, light-absorbing regions are provided periodically along the guiding direction of the stimulated emission light. Thus, the periodical change of gain is effectively provided in the device structure.
More particularly, in the GC-DFB-LD 900, an n-GaAs current confinement layer 902 including a stripe-shaped opening 902a is formed on a p-GaAs substrate 901. A p-AlGaAs cladding layer 904 is formed so as to cover the current confinement layer 902 and a portion of the surface of the substrate 901 which is exposed through the opening 902a. The opening 902a of the current confinement layer 902 serves as a current confinement groove 903.
The un-AlGaAs active layer 905 is formed on the cladding layer 904. An n-GaAs light absorbing layer 907 is formed on the active layer 905 via an n-AlGaAs buffer layer 906. The thickness of the light absorbing layer 907 is periodically changed (i.e., uneven) along the longitudinal direction of the stripe-shaped opening 902a.
An n-AlGaAs cladding layer 908 with a flat top surface is formed on the light absorbing layer 907. The surface of the cladding layer 908 is covered by an n-GaAs cap layer 909. An n-electrode 910b is formed on the surface of the n-GaAs cap layer 909. On the other hand, a p-electrode 910a is formed on the bottom surface of the p-GaAs substrate 901.
Next, a method for producing the GC-DFB-LD 900 having such a structure will be described.
First, the n-GaAs current confinement layer 902 is grown on the p-GaAs substrate 901 by using a liquid phase epitaxy method. Then, the current confinement layer 902 and a surface region of the substrate 901 are selectively etched so as to form the current confinement groove 903.
Next, the p-AlGaAs cladding layer 904 is grown by using the liquid phase epitaxy method so as to entirely cover the upper surfaces of the current confinement layer 902 and the current confinement groove 903. Herein, the upper surface of the formed cladding layer 904 is formed to be flat. Subsequently, the un-AlGaAs active layer 905, the n-AlGaAs buffer layer 906 and the n-GaAs light absorbing layer 907 are grown in this order by using the liquid phase epitaxy method.
Then, the light absorbing layer 907 is selectively etched by using a dual light beam interference exposure method and a wet etching so as to form a diffraction grating with a pitch of about 240 nm in the surface of the light absorbing layer 907.
Next, the n-AlGaAs cladding layer 908 and the n-GaAs cap layer 909 are grown in this order on the light absorbing layer 907. Then, the p-electrode 910a and the n-electrode 910b are formed on the bottom surface of the substrate 901 and on the upper surface of the cap layer 909, respectively. In this way, the GC-DFB-LD 900 is produced.
In the GC-DFB-LD 900 having such a structure, an effective periodical change occurs in the gain of the stimulated emission light generated in the active layer 905 due to the light absorbing layer 907 including the diffraction grating. Therefore, gain coupling of the stimulated emission light occurs and laser oscillation with a single wavelength is thereby obtained.
However, in the GC-DFB-LD 900, the refractive index differs between the GaAs light absorbing layer 907 and the AlGaAs cladding layer 908. Therefore, while there occurs the intended gain coupling, there also occurs undesirable refractive index coupling of the stimulated emission light due to the periodical change of refractive index. As a result, the excellent laser oscillation characteristics based on the gain coupling are deteriorated.
When the refractive index coupling and the gain coupling of stimulated emission light coexist in a DFB-LD, as in the aforementioned GC-DFB-LD 900, such a DFB-LD is called a "partial GC-DFB-LD". On the other hand, when a distributed feedback of stimulated emission light is provided exclusively based on the gain coupling, such a GC-DFB-LD is called a "pure GC-DFB-LD". These two GC-DFB-LDs are distinguished from each other.
Japanese Laid-Open Patent Publication No. 5-29705 discloses a distributed feedback semiconductor laser diode which is made in view of solving the above-described problem, i.e., the deterioration of the excellent laser oscillation characteristics to be obtained through the gain coupling because of the coexistence of the gain coupling and the refractive index coupling in a GC-DFB-LD.
In the distributed feedback semiconductor laser diode described in this publication, a structure for cancelling out the periodical change of refractive index is introduced. More specifically, an absorption-based diffraction grating having a structure as shown in FIG. 10A is formed in the vicinity of an active layer.
This structure includes a lower transparent layer 13 which is transparent for stimulated emission light generated in the active layer. A light absorbing layer 11 is formed on the lower transparent layer 13 in a periodical manner at constant pitches along the guiding direction as shown in the figure. Grooves 13a are provided in the surface of the transparent layer 13 at positions between adjacent portions of the light absorbing layer 11. An upper transparent layer 12, which is also transparent for the stimulated emission light, is formed so as to cover the light absorbing layer 11 and the grooves 13a in the lower transparent layer 13.
In this structure, the refractive index of the upper transparent layer 12 is set to be greater than that of the lower transparent layer 13, while the refractive index of the light absorbing layer 11 is set to be greater than that of the upper transparent layer 12. More particularly, FIG. 10B shows the periodical changes of refractive index along line A-A' and line B-B' of FIG. 10A. As shown in FIG. 10B, the refractive index change along line A-A' is: high (H)--low (L)--high (H)--. . . , whereas the change along line B-B' is: low (L)--high (H)--low (L)--. . . ; in other words, the periodical change of the magnitude of refractive index is opposite between line A-A' and line B-B'.
In the description hereinafter, the period of the refractive index change and the period of the gain change are referred to also as the "phase of a periodical change of refractive index" and the "phase of a periodical change of gain", respectively. Accordingly, when the refractive index and the gain correspondingly change between a high value and a low value (i.e., a gain is high where a refractive index is high), such a situation will be described also as "the phase of the periodical change of refractive index matches the phase of the periodical change of gain" or "they have the same phases with each other". When the refractive index and the gain oppositely change between a high value and a low value (i.e., a gain is low where a refractive index is high), such a situation will be described also as "the phase of the periodical change of refractive index is opposite to the phase of the periodical change of gain" or "they have the opposite phases with each other".
When the profile of the interface between the light absorbing layer 11 and the upper transparent layer 12 as well as the profile of the interface between the upper and lower transparent layers 12 and 13 are respectively precisely adjusted, an absorption-based diffraction grating is realized. In the absorption-based diffraction grating, the periodical change of refractive index is cancelled out as a whole. Consequently, a structure of the pure GC-DFB-LD, in which a distributed feedback is provided purely based on the gain coupling, can be obtained.
Moreover, in a semiconductor laser diode disclosed in Japanese Laid-Open Patent Publication No. 4-155987, the structure for cancelling out the periodical change of refractive index in the absorption-based diffraction grating is utilized in a gain-based diffraction grating. This semiconductor laser diode includes a gain-based diffraction grating in which the periodical change of refractive index is cancelled out by providing regions having the opposite phases of periodical change of refractive so as to adjoin each other.
However, the above-described conventional DFB-LD structures have the following disadvantages.
The device structure of the conventional partial GC-DFB-LD realizes a much higher yield of device structures realizing laser oscillation with a single wavelength, as compared to the device structure of the IC-DFB-LD. However, when stimulated emission light is directly modulated at a high speed in this device structure, there occurs a discontinuous shift (mode hopping) of wavelength and simultaneous oscillation of a plurality of wavelengths (multi-mode oscillation). This has been a drawback to application of this device structure.
In the case of the conventional pure GC-DFB-LD where the periodical change of refractive index is cancelled out, even a slight shift in the shape, depth, etc. of diffraction grating causes a great imbalance between the previously explained periodical changes of refractive index along line A-A' and line B-B' (see FIGS. 10A and 10B). In such a case, it is difficult to completely cancel the periodical change of refractive index. Thus, in order to provide a structure which cancels out the periodical change of refractive index in a satisfactory manner, a highly precise processing accuracy and reproducibility are required. However, it is extremely difficult to produce such a structure.
For example, the diffraction grating for a DFB-LD requires a large number of extremely small gratings (i.e., convex/concave portions) with a pitch of about 100 nm to 400 nm. Generally, it is very difficult to form such a large number of convex/concave portions while controlling various processing conditions so that the produced convex/concave portions are in the intended shape.
Therefore, with the pure GC-DFB-LD produced by the conventional techniques, it is practically impossible to completely cancel out the adverse affect of the refractive index coupling on a distributed feedback of stimulated emission light. As a result, the produced semiconductor laser diode operates as a partial GC-DFB-LD. Thus, it is nearly impossible with the conventional techniques to practically produce a desirable pure GC-DFB-LD.